A laser scanner is one type of coordinate measurement device typically used for non-contact optical scanning of many different types of relatively large closed or open spaces or objects, for example, interior spaces of buildings, industrial installations and tunnels, or exterior shapes of planes, automobiles or boats. Laser scanners can be used for many different purposes, including industrial applications and accident reconstruction. A laser scanner optically scans and measures the environment around the laser scanner by emitting a rotating laser beam and detecting the laser beam as it is reflected back from the various objects in its path. Laser scanners typically collect a variety of data points with respect to the environment, including distance information for each object in its surrounding environment, a grey scale value (i.e., a measure of the intensity of light) for each distance measurement value, and coordinates (e.g., x, y, and z) for each distance measurement value. This scan data is collected, stored and sent to a processor that is typically remote from the laser scanner, where the data is processed to generate a three dimensional (3D) scanned image of the scanned environment with measurements. In order to generate the 3D scanned image, at least four values (x, y, z coordinates and grey scale value) are collected for each scanned data point.
Many contemporary laser scanners also include a camera mounted on the laser scanner for gathering digital images of the environment and for presenting the digital images to an operator of the laser scanner. The images can be oriented together with the scanned data to provide a more realistic image of the object being scanned. By viewing the images, the operator of the scanner can determine the field of view of the scanned data, and can adjust the settings on the laser scanner if the field of view needs adjusting. In addition, the digital images may be transmitted to the processor to add color to the 3D scanned image. In order to generate a 3D color scanned image, at least six values (x, y, z coordinates; and red value, green value, blue value or “RGB”) are collected for each data point. Examples of laser scanners are disclosed in U.S. Pat. No. 7,193,690 to Ossig et al.; U.S. Pat. No. 7,430,068 to Becker et al.; and U.S. Published Patent Application No. US2010/0134596 to Becker; each being incorporated by reference herein.
Another type of coordinate measurement device is a laser tracker, which measures the 3D coordinates of a certain point by sending a laser beam to the point, where the laser beam is typically intercepted by a retroreflector target. The laser tracker finds the coordinates of the point by measuring the distance and the two angles to the target. The distance is measured with a distance-measuring device such as an absolute distance meter (ADM) or an interferometer. The angles are measured with an angle-measuring device such as an angular encoder. A gimbaled beam-steering mechanism within the instrument directs the laser beam to the point of interest. The retroreflector may be moved manually by hand, or automatically, over the surface of the object. The laser tracker follows the movement of the retroreflector to measure the coordinates of the object. Exemplary laser trackers are disclosed in U.S. Pat. No. 4,790,651 to Brown et al., incorporated by reference herein; and U.S. Pat. No. 4,714,339 to Lau et al. The total station, which is most often used in surveying applications, may be used to measure the coordinates of diffusely scattering or retroreflective targets. The total station is closely related to both the laser tracker and the scanner.
A common type of retroreflector target is the spherically mounted retroreflector (SMR), which comprises a cube-corner retroreflector embedded within a metal sphere. The cube-corner retroreflector comprises three mutually perpendicular mirrors. The apex of the cube corner, which is the common point of intersection of the three mirrors, is located at the center of the sphere. It is common practice to place the spherical surface of the SMR in contact with an object under test and then move the SMR over the surface of the object being measured. Because of this placement of the cube corner within the sphere, the perpendicular distance from the apex of the cube corner to the surface of the object under test remains constant despite rotation of the SMR. Consequently, the 3D coordinates of the object's surface can be found by having a tracker follow the 3D coordinates of an SMR moved over the surface. It is possible to place a glass window on the top of the SMR to prevent dust or dirt from contaminating the glass surfaces. An example of such a glass surface is shown in U.S. Pat. No. 7,388,654 to Raab et al., incorporated by reference herein.
A gimbal mechanism within the laser tracker may be used to direct a laser beam from the tracker to the SMR. Part of the light retroreflected by the SMR enters the laser tracker and passes onto a position detector. The position of the light that hits the position detector is used by a tracker control system to adjust the rotation angles of the mechanical azimuth and zenith axes of the laser tracker to keep the laser beam centered on the SMR. In this way, the tracker is able to follow (track) the SMR as it is moved.
Angular encoders attached to the mechanical azimuth and zenith axes of the tracker may measure the azimuth and zenith angles of the laser beam (with respect to the tracker frame of reference). The one distance measurement and two angle measurements performed by the laser tracker are sufficient to completely specify the three-dimensional location of the SMR.
As mentioned, two types of distance meters may be found in laser trackers: interferometers and absolute distance meters (ADMs). In the laser tracker, an interferometer (if present) may determine the distance from a starting point to a finishing point by counting the number of increments of known length (usually the half-wavelength of the laser light) that pass as a retroreflector target is moved between the two points. If the beam is broken during the measurement, the number of counts cannot be accurately known, causing the distance information to be lost. By comparison, the ADM in a laser tracker determines the absolute distance to a retroreflector target without regard to beam breaks, which also allows switching between targets. Because of this, the ADM is said to be capable of “point-and-shoot” measurement. Initially, absolute distance meters were only able to measure stationary targets and for this reason were always used together with an interferometer. However, some modern absolute distance meters can make rapid measurements, thereby eliminating the need for an interferometer. Such an ADM is described in U.S. Pat. No. 7,352,446 to Bridges et al., incorporated by reference herein. The distances measured by interferometers and absolute distance meters are dependent on the speed of light through air. Since the speed of light varies with air temperature, barometric pressure, and air humidity, it is common practice to measure these quantities with sensors and to correct the speed of light in air to obtain more accurate distance readings. The distances measured by total stations and scanners also depend on the speed of light in air.
In its tracking mode, the laser tracker automatically follows movements of the SMR when the SMR is in the capture range of the tracker. If the laser beam is broken, tracking will stop. The beam may be broken by any of several means: (1) an obstruction between the instrument and SMR; (2) rapid movements of the SMR that are too fast for the instrument to follow; or (3) the direction of the SMR being turned beyond the acceptance angle of the SMR. By default, following the beam break, the beam may remain fixed at the point of the beam break, at the last commanded position, or may go to a reference (“home”) position. It may be necessary for an operator to visually search for the tracking beam and place the SMR in the beam in order to lock the instrument onto the SMR and continue tracking.
Some laser trackers include one or more cameras. A camera axis may be coaxial with the measurement beam or offset from the measurement beam by a fixed distance or angle. A camera may be used to provide a wide field of view to locate retroreflectors. A modulated light source placed near the camera optical axis may illuminate retroreflectors, thereby making them easier to identify. In this case, the retroreflectors flash in phase with the illumination, whereas background objects do not. One application for such a camera is to detect multiple retroreflectors in the field of view and measure each retroreflector in an automated sequence. Exemplary systems are described in U.S. Pat. No. 6,166,809 to Pettersen et al., and U.S. Pat. No. 7,800,758 to Bridges et al., incorporated by reference herein.
Some laser trackers have the ability to measure with six degrees of freedom (DOF), which may include three coordinates, such as x, y, and z, and three rotations, such as pitch, roll, and yaw. Several systems based on laser trackers are available or have been proposed for measuring six degrees of freedom. Exemplary systems are described in U.S. Pat. No. 7,800,758 to Bridges et al., U.S. Pat. No. 5,973,788 to Pettersen et al., and U.S. Pat. No. 7,230,689 to Lau.
It is desirable to provide a laser scanner or a laser tracker with one or more projectors, with each projector projecting visual information in the form of images and/or data (e.g., CAD data or scanned point cloud data) onto various surfaces. The projected visual information may, for example, be of a type such as to provide guidance to an operator, such as written instructions, highlighted points to be measured, indicated areas where data are to be taken, and real time feedback on the quality of the data.